Implantable medical systems including implantable medical devices (IMD) and associated implantable medical leads provide functions such as stimulation of muscle or neurological tissue and/or sensing of physiological occurrences within the body of a patient. Typically, the IMD is installed in a subcutaneous location that is accommodating and relatively accessible for implantation. For instance, to provide stimulation near the spine or pelvis, the IMD may be installed in a pocket located on the abdomen or upper buttocks region of the patient. The implantable medical lead is installed, either through a percutaneous procedure or a surgical procedure, depending upon the type of lead that is necessary.
Once installed, the lead extends from the stimulation site to the location of the IMD. The separation of the stimulation site to the location of the IMD varies, but may typically range from about 20 cm to about 100 cm. For relatively lengthy separation, if a lead of adequate length is unavailable then a lead extension may be implanted to span from the IMD to a proximal end of the implantable lead.
The implantable medical lead includes connector rings on a proximal end and electrodes on a distal end, and conductive filars interconnecting the connector rings to the electrodes. The lead includes a jacket, often made of a flexible but biocompatible polymer, and the filars are insulated from the body tissue by the jacket. However, the filars are not insulated by the jacket from the presence of electromagnetic radiation. Electromagnetic radiation in the radio frequency (RF) spectrum induces currents into the filars and thus presents current at the electrode that is unintended. In the patient's normal daily experience, the level of RF radiation that is encountered is at a negligible level, and there is no danger of heating of tissue by the unintended current that may result.
RF radiation poses a risk to tissue in contact with the electrodes when the intensity is significantly higher than the background levels. The surface area of each electrode is relatively small so that a small amount of tissue must dissipate a potentially large amount of induced current. In particular, if the patient is exposed to the RF radiation from a magnetic resonance imaging (MRI) scan, there is a high probability that tissue damage at the stimulation site(s) can occur. This tissue damage may be very dangerous, particularly so for neurological tissue. Therefore, patients with IMDs are typically not permitted to have a body coil MRI scan for at least these reasons.